The present invention relates to a process for the production of high surface area tantalum and/or niobium powders.
An important application of tantalum and niobium powders is to be used in manufacturing the electrolytic capacitors. The manufacture of tantalum or niobium solid electrolytic capacitor is typically comprised of: compressing the tantalum or niobium powder to form a pellet with an embedded tantalum or niobium lead wire, sintering the pellet to a porous body, subsequently forming a continuous dielectric oxide thin film on the surface of the porous pellet by anodizing, coating the oxide film with cathode material, and the final enveloping operation. The capacitance of the capacitor is depended on the surface area of tantalum and/or niobium powders. The higher surface area of the powder used, the higher capacitance of the capacitors may be obtained. Leakage current of the capacitor is also an important parameter in evaluating the quantity of the capacitor. As the impurities degrade the dielectric properties of the oxide film, low leakage current capacitors can be obtained by using the tantalum and/or niobium powders with high purity.
There are two kinds of methods for the production of tantalum and niobium powders in the prior art. One method is electronic beam method, in which the electronic beam melted tantalum or niobium ingot was hydrogenated and then pulverized, and the obtained powder has high purity, but with low surface area, therefore result in low capacitance of the capacitors made with these powders. The other method is chemical reduction method, in which the compound containing tantalum or niobium is reduced by reducing agent, and the obtained powder is leached with acids and water.
The typical process for producing tantalum powder is reducing potassium fluorotantalate (K2TaF7) with sodium as published in U.S. Pat. No. 3,012,877. As summarized in WO 91/18121, in order to obtain high surface area tantalum powder via chemical reduction method, a certain amount of diluent, such as alkali metal halides selected from NaCl, KCl, KF and NaF was added in the raw materials to be reduced. However, if higher surface area of the powder is required, more diluent should be used in said method for producing tantalum powder. Unfortunately, when more diluent is used, tantalum powder will be contaminated with more impurities and the yield will also be decreased. Moreover, as the surface area of the powder reaches to a certain extent, it can hardly be increased even if more diluent is used in the reduction reaction. As a result, tantalum powder obtained by reducing K2TaF7 with sodium in industry usually has a surface area of between 0.2xcx9c2.0 m2/g, it is almost impossible to produce higher surface area tantalum powder by the chemical reduction method.
U.S. Pat. No. 6,136,062 disclosed a method of producing niobium and/or tantalum powders by reducing corresponding niobium and/or tantalum oxides with magnesium metal, wherein the first reduction stage is carried out as far as an average composition corresponding to (Nb, Ta) Ox (x=0.5xcx9c1.5), and before the second reduction stage, the reduction product from die first stage is freed from excess reducing metals and alkaline earth metal oxides formed in the reduction by washing with mineral acids. Although this process can produce high surface area niobium and/or tantalum powders, the disadvantages are that the amount of reducing agent used is too much, that the amount of acid needed is too much. Moreover, this process includes two stages of reduction, and the degree of reduction of the first stage must be critically controlled. Therefore, the process is complicated and low efficiency.
In order to solve the problem described above, the present inventors developed an economic process of producing high surface area tantalum and/or niobium powders, in which reduction of tantalum and/or niobium oxides are carried out with an alkali metal and at least one halide selected from the group consisting of halides of Mg, Ca, Sr, Ba and Ce.
The object of the invention is to provide an economic process for the production of high surface area tantalum and/or niobium powders via the reduction of corresponding tantalum and/or niobium oxides, wherein the reduction is carried out by reacting the tantalum and/or niobium oxides with at least one metal halide and an alkali metal at elevated temperature so as to form the tantalum and/or niobium powders, said metal halide is selected from the group consisting of halides of Mg, Ca, Sr, Ba and Ce.
According to a preferred embodiment of the present invention, at least one alkali metal halide is further used in the reduction as a diluent, said alkali metal halide may be selected from sodium chloride, potassium chloride, lithium chloride, potassium fluoride, sodium fluoride.
According to an embodiment of the present invention, said process comprises charging said at least one metal halide selected from the group consisting of halides of Mg, Ca, Sr, Ba and Ce, the alkali metal, the tantalum and/or niobium oxides, and the optional at least one alkali metal halide in a reactor, heating the reactor to elevated temperature so that the tantalum and/or niobium oxides are reduced to tantalum and/or niobium powders.
According to another embodiment of the present invention, said process comprises charging said at least one metal halide selected from the group consisting of halides of Mg, Ca, Sr, Ba and Ce, and the optional alkali metal halide in a reactor, heating the reactor to elevated temperature to form a molten bath, and then metering required amount of tantalum and/or niobium oxides and alkali metal to the molten bath while controlling the temperature of the reactor so that the tantalum and/or niobium oxides are reduced to tantalum and/or niobium powders.
According to the process of the present invention, the reduction is usually carried out at a temperature in the range of 400-1200xc2x0 C., preferably in the range of 600-1000xc2x0 C. for about 20-300 minutes so that the reduction can be carried out completely.
According to the present invention, the said alkali metals are used as reducing agent, said alkali metals are preferably selected from sodium, potassium and lithium, sodium and/or potassium are particularly preferred. The amount of alkali metals used is 1.0 to 1.3 times of the stoichiometric amount for reducing the tantalum and/or niobium oxides. According to the present invention, the halide selected from halides of Mg, Ca, Sr, Ba and Ce is used as both a diluent and as an indirect reducing agent, wherein halides of Mg and Ca are preferred. The mole amount of said metal halides used is 0.5 to 8.0 times of the mole amount of the alkali metal used.
The tantalum and/or niobium oxides used for the present invention may be any tantalum and/or niobium oxides or their mixture which is capable of being reduced to tantalum and/or niobium metal, for example, Ta2OX (xxe2x89xa65), Nb2OX (xxe2x89xa65). They are generally available as Ta2O5 and Nb2O5.
In order to obtain high surface area tantalum and/or niobium powders as well as the sintered anodes formed from them, according to the present invention, a dopant containing N, P, S, B or Si can be further added to the above raw materials used in the reduction reaction, and/or added during the reduction reaction, and/or added after the reduction reaction.
According to the present invention, the reduction is usually carried out in a closed reactor made from refractory alloy. In order to make the reactant dispersing homogeneously in the molten salts and to prevent from local overheating, the reactor is preferably equipped with a stirrer. In addition, the reactor is preferably equipped with a heating device and a cooling device so as to control the temperature of the reactor. The said reactor as well as the stirrer, the heating device and cooling device can be any equipment that is well known to the skill in the art.
According to the process of the present invention, the reduction is carried out under the inert atmosphere, for example argon and/or nitrogen atmosphere. The reactor is maintained under inert gas until that the mass in the reaction vessel is cooled to ambient temperature.
After the reduction, the reduction product was cooled and taken out of the reactor, and crushed and then washed with mineral acid solution and de-ionized water to remove the excess alkali metals, alkali metal halides, the halides and oxides of Mg, Ca, Sr, Ba and Ce, so as to obtain the agglomerates of tantalum and/or niobium powders. Suitable mineral acid solution for washing the product is at least one of hydrochloric acid, nitric acid, sulphuric acid, hydrofluoric acid and hydrogen peroxide or mixture thereof
The tantalum and/or niobium powders obtained as said above are dried with common method. The dried tantalum and/or niobium powders are screened with a 40-100 mesh sieve, the fine powders passing the sieve are subjected to chemical analysis and physical properties test.
Without being bound to a particular theory, it is believed that the reduction of the tantalum and/or niobium oxides in the present invention is carried out as:
MxOy+2yMa+yMeR2=xM+yMeO+2yMaR
Wherein, M is Ta and or Nb; Ma is alkali metal, preferably selected from at least one of Na, K and Li; Me is selected from at least one of Mg, Ca, Sr, Ba and Ce; R are halide ions. MxOy express the oxides of tantalum and/or niobiwn with five valences or less than five valences. So it can be regarded that the alkali metals are used as reducing agent, and the halides of Mg, Ca, Sr, Ba and Ce are used both as indirect reducing agent and as a diluent.
The tantalum and/or niobium powders produced according to the process of the present invention are comprised of porous agglomerates comprising many primary particles. The tantalum powder has a BET surface area of 1-30 m2/g, and the niobium powder has a BET surface area of 1-40 m2/g, and the primary particle size of these powder is in a range of 10-250 nm, and preferable 20-100 nm. The oxygen content of the tantalum and/or niobium powders produced by the process of the present invention is in the range of 4000-80000 ppm, and the alkali metal content is less than 20 ppm, preferable less than 5 ppm.
The tantalum and/or niobium powders produced by the process of the present invention can be subjected to further refinery operation such as doping, heat agglomeration and deoxidization. The technologies of doping, heat agglomeration and deoxidization are well known in the art. The doping can be carried out by treating tantalum and/or niobium powders produced by process of the present invention with a dopant comprising N, P, S, B or Si. The deoxidization can be carried out by heating the tantalum and/or niobium powders produced by the process of the present invention with a reducing agent such as magnesium and calcium, and optional diluent such as alkali metal halide. The heat agglomeration can be carried out in a vacuum furnace at a temperature of 700-1400xc2x0 C. for a period of 10-120 minutes. The said agglomerated tantalum and/or niobium powders usually have a medium particle size (D50) of 40-300 xcexcm, preferable 40-200 xcexcm, and the agglomerated particles have good flowability, and are suitable to manufacture capacitors.